Catalytic converters are conventionally included in the exhaust system of automotive vehicles to reduce the level of pollutants discharged to the air. While it is generally believed that the catalytic converters used today perform satisfactorily once their light-off temperature is reached, a pollution problem exists during the light-off period. For example, it has been determined that 90% of the pollutants exhausted to the atmosphere from an exhaust system which includes a catalytic converter are formed during the light-off period. As used herein, the light-off temperature is the temperature at which a catalytic converter catalyzes the reaction that takes place in the converter with the exhaust gases. The catalytic light-off period is the time required for the catalytic converter to reach its light-off temperature.
If the heat of the exhaust gases, which can reach temperatures as high as 1800.degree. F. in turbo-charged automobiles, can be retained for a longer period of time than in conventional exhaust systems, the time required for the light-off temperature to be reached will be reduced. This would then reduce the duration of high pollution, and in turn reduce the amount of pollutants released to the atmosphere.
Attempts have been made in the past to develop insulated exhaust systems. Double exhaust pipes have been suggested, comprising spaced inner and outer pipes. Although this reduces the amount of heat loss, it is not enough to appreciably retain heat at the level required for optimum catalytic converter operation.
Another suggestion is found in U.S. Pat. No. 4,345,430, issued to Pallo et al. In that patent a double pipe system comprised of inner and outer corrugated metal tubes is disclosed. In addition, the use of insulation between the inner and outer tubes is suggested. Various types of insulation materials capable of withstanding temperatures up to 1600.degree. F. are suggested in the patent. At the temperature requirements of modern automobiles and catalytic converters, however, refractory fiber insulation is the most practical choice of insulation to be used.
Although refractory fiber is capable of resisting the high temperatures to which it would be exposed and of providing the necessary degree of insulation, it is a very fragile material. When used in an insulated exhaust system such as that disclosed in U.S. Pat. No. 4,345,430 it was found that the physical stresses to which it was exposed during use caused it to be reduced to small dust-like particles. In this condition it was no longer able to provide satisfactory insulation. Obviously, if refractory fibers are to be used in an insulated exhaust system they must be be capable of resisting degradation.
Moreover, a manufacturing process must be found to efficiently produce a length of double pipe which has refractory fiber insulation in the space between the pipes. This problem was addressed in U.S. Pat. No. 4,345,430, but the proposed solution was not found to be practical. It was suggested to form a first relatively small tube of spirally wound corrugated metal and to spirally wind a layer of refractory fiber felt onto the tube. To encase the insulated tube in another larger tube, it was suggested to spirally wind a second corrugated metal tube directly over the insulated tube. This method is too difficult to regulate and the required machinery to carry it out would be too complicated and expensive. Excessive pressure on the felt as it is being wound about the corrugated tube tends to tear the felt, and it is difficult to precisely wind the felt so as to avoid overlaps and gaps between windings. Further, maximum efficiency in producing the outer corrugated tube cannot be attained by winding adjacent corrugated strips about a previously formed insulated pipe while at the same time forming seams from the adjacent edges of the strips.
A much simpler and more reliable method of manufacture is needed for the commercial production of an insulated exhaust pipe.